In certain semiconductor devices, a structure is required in which a highly doped layer is disposed adjacent to a layer with a substantially lower doping. In certain cases, this arrangement can be problematic as such structures are potentially susceptible to dopant migration, via diffusion and other mechanisms. For example, when a first layer of silicon doped with boron (B) is formed adjacent to and located underneath a second layer of silicon that is undoped or has a significantly lower B doping concentration than the first layer, unwanted migration of B into the second layer can occur when a thermal treatment is applied during fabrication of the semiconductor device. Similar results have been observed for other dopants in silicon, such as phosphorus (P) and arsenic (As). As a result, the abrupt doping transition between differentially doped layers may be degraded or compromised by subsequent thermal processing. In a worst case, the dopant could diffuse completely through the second layer, altering the undoped nature of the second layer in its entirety. In either case, the electrical characteristics of the semiconductor device can be significantly altered when dopant migration occurs.
In an effort to avoid migration of highly mobile dopants, into undoped regions, development efforts have primarily focused on: (1) reducing the thermal budget during semiconductor device manufacturing processes and (2) the implantation of additional species to form blocking regions to inhibit the migration of dopants. For example, carbon (C) implants activated into substitutional lattice sites (such as by first performing a pre-amorphization followed by C implantation and recrystallization anneal) have been utilized to suppress migration of B and P in silicon.
Both of these efforts (lower thermal budgets and migration inhibitors) have met with some success, but still have drawbacks. With respect to thermal budget reductions, at least some thermal treatments will always be required and therefore the amount of thermal budget reduction is always limited. Additionally, variations in the manufacturing process can limit the effectiveness of the reduced thermal budget. In particular, defects and interstitial/vacancy pairs generated during normal semiconductor device processing can result in migration, even when a lower thermal budget is applied. Moreover, even if thermal budgets are accurately controlled, small amounts of unwanted dopant diffusion can still have a pronounced effect on devices with reduced dimensions or devices designed with low doping concentrations. In addition, some thermal steps can simply lose effectiveness if the temperature is lowered beyond a critical minimum value.
With respect to ion implantation of a migration inhibitor such as carbon, one potential issue is that there can be inaccuracies in the placement of the carbon species, resulting in some dopants being placed in undesirable locations or atoms missing from locations where the dopant diffusion protection is needed. As a result, some migration can occur due to the ineffectiveness of the ion implantation process leading to the imperfect placement of the species intended to suppress migration. Further, ion implantation processes can introduce additional contaminating impurities into the semiconductor device and additional implant damage. Either of these can adversely affect device performance. Moreover, the extra implantation steps can introduce additional costs to the manufacturing process.
In view of the foregoing, there is a need for an alternate approach for suppressing dopant migration in semiconductor devices.